1. Field of the Invention
The present invention relates to a thermal print head and method for making the same. More particularly, the present invention relates to a thermal print head and method for making the same in which a metal film and a cooling compound are inserted between a resistance substrate and a cooling board.
2. Description of the Prior Art
Thermal recording is a technique by which characters or graphics are recorded on white thermal paper based on the characteristic of white thermal paper that only heated portions are turned black. A thermal printer is a machine to which the above-mentioned technique of the thermal recording is applied. The thermal printer uses a thermal print head on which heating elements for converting electrical energy into heat energy are formed linearly like a row of dots.
When an operator using the thermal printer prints characters or graphics using the thermal printer, the heating elements are selectively heated to generate heat based on data inputted as an electrical signal while the thermal print head contacts a medium such as the thermal paper. The generated heat is applied to the thermal paper to record the characters or graphics as a series of dots. The image is formed by sequentially applying lines of data, i.e., heated dots, to the thermal paper.
A conventional thermal print head will be described in detail with reference to accompanying drawings hereinafter. FIG. 1 is a sectional view of a conventional thermal print head, and FIG. 2 is a perspective view illustrating a portion corresponding to a reference number, 10 in FIG. 1.
Referring to FIG. 1, the conventional thermal print head comprises a resistance substrate 1 connected to a driving substrate 30. The resistance substrate 1 is preferably made of ceramics or the like and has a plurality of heating elements 2 formed on a front surface. These heating elements 2 generate heat when they are supplied with current. The driving substrate 30 has a driving integrated circuit 21 formed on it, which separately drives the resistance substrate 1 and the heating elements 2.
Referring to FIG. 2, the heating elements 2 are linearly formed in a predetermined direction like a row of dots, and are selectively driven and heated selectively to generate heat on the thermal paper.
A cooling board 3 made of metal with high thermal conductivity is adhered to a rear surface of the resistance substrate 1 by an adhesive 4. The cooling board acts to dissipate heat generated by the heating elements 2. As shown in FIG. 1, a part of the cooling board 3 is connected to a part of the driving substrate 30 by the adhesive 4, thereby supporting the driving substrate 30. A connector 40 supports the remainder of the driving substrate 30.
A protector 22 protects the driving integrated circuit 21 by covering the driving integrated circuit 21, and a cover 20 provides further protection by covering over the protector 22. The cover 20 is attached to the driving substrate 30 by a screw 50.
A problem arises, however, when the thermal prim head is used at room temperature. Then, a surface temperature of the resistance substrate 1 can rise to as high as 200.degree. C. due to the heat generated from the heating elements 2. Accordingly, heat accumulates and any excess heat generated from the heating elements 2 that does not contribute to printing must be dissipated. Otherwise, the excess heat might distort current and future prim jobs. When the dissipation of heat is not carried out efficiently, uneven printing contrast occurs due to heat accumulation. Accordingly, the cooling board 3 is attached to the lower part of the resistance substrate 1 to enhance heat dissipation.
Generally, a double-sided tape is used as the adhesive 4 to attach the cooling board 3 to the resistance substrate 1. Double-sided tape is used because the resistance substrate 1 and the cooling board 3 differ significantly in their coefficients of thermal expansion. If the resistance substrate 1 and the cooling board 3 were fixed directly, they would be warped by the heat during printing in the same way that a bimetal is warped by changes in its ambient temperature. The use of double-sided tape reduces the stress caused by the difference in coefficient of thermal expansion of the resistance substrate 1 and the cooling board 3 and thereby prevents the warping of the resistance substrate 1. In addition, using double-sided tape also simplifies production of the printer by making it easier to join the cooling board to the resistance substrate.
Double-sided tape is not without its problems, however. When the resistance substrate 1 and the cooling board 3 are joined by double-sided tape, heat generated by the resistance substrate 1 is not sufficiently dissipated into the cooling board 3. This occurs because the thermal conductivity of the double-sided tape is generally small, for example, less than 0.5.times.10.sup.-3 cal/cm-sec-.degree.C. The incomplete heat dissipation caused by using the double-sided tape results in uneven contrast and smearing of the printed image because too much heat remains in the resistance substrate 1. Accordingly, the double-sided tape cannot be used in high speed printers, color printers, high speed label printers, and the like, which require greater heat dissipation.
To overcome the above-mentioned disadvantage, a cooling compound can be used in place of some of the adhesive 4. Unfortunately, the thickness of the cooling compound can differ locally along the resistance substrate 1 because the manufacturing process results in areas having a difference in height on the order of tens to hundreds of microns. The uneven thickness in turn causes uneven cooling and therefore uneven contrast in the primed image.
To solve the disadvantages of using a cooling compound, the following thermal print head has been proposed, as shown in FIGS. 3 and 4 and described below.
Referring to FIG. 3, a cooling material or a cooling compound 6, in place of the adhesive 4, is inserted between the cooling board 3 and the area of the rear surface of the resistance substrate 1 corresponding to the area on the front surface where the heating elements 2 are formed. Two long grooves 5 are formed in the cooling board 3 in the direction in which the heating elements 2 are arranged. The adhesive 4 is positioned at both sides of the cooling compound 6, bordering on the grooves 5.
The cooling compound 6 is preferably a mixture of fine particles of aluminum oxide or zinc oxide in a size of 1 .mu.m or less and, for example, silicon oil. It is preferably a viscous, greasy matter with a thermal conductivity in the range of 1.5 to 3.0.times.10.sup.-3 cal/cm-sec-.degree.C. To compare, the preferred cooling compound 6 possesses a thermal conductivity of 3 to 6 times higher than that of the adhesive 4.
After applying the cooling compound 6 between the long grooves 5 on the cooling board 3, the resistance substrate 1 is pressed together with the cooling board 3. As this is done, the cooling compound 6 is compressed and spread widely on the cooling board 3. The long grooves 5 are provided to hold the overflowing cooling compound 6.
As shown in the thermal head print illustrated in FIG. 4, a long central groove 5' may be formed on a part of the cooling board 3 corresponding to the area where the heating elements 2 are linearly formed. At both sides of the long central groove 5', grooves 5 are formed, and an adhesive with high thermal conductivity is inserted in them.
The thermal print head is manufactured by pressing the resistance substrate 1 and the cooling board 3 together at a high temperature with adhesive 6' in between. While the resistance substrate 1 and the cooling board 3 are pressed together, and the grooves 5 serve to hold any adhesive material displaced in the pressing process.
However, in cases of the conventional thermal print head as illustrated in FIGS. 3 and 4, local heat accumulation can occur in the resistance substrate 1 when an air bubble, having a high thermal conductivity, is formed in the adhesive 6', or when the cooling compound 6 and the adhesive 4 overlap one another when the resistance substrate 1 and the cooling board 3 are adhered. In addition, it is also disadvantageous that insufficient heat dissipation occurs in high speed printing through the use of only the cooling compound 6 or the adhesive 6'.